Attack Chain

1. A user receives a spearphishing email with a malicious PDF attachment (T1566.001).
2. The user opens the PDF file using a vulnerable PDF reader application (e.g., Acrobat Reader, Foxit Reader).
3. The PDF file exploits a vulnerability in the PDF reader, triggering the execution of embedded JavaScript or shell commands (T1203, T1204.002).
4. The exploited PDF reader process (AcroRd32.exe, Acrobat.exe, FoxitPhantomPDF.exe, FoxitReader.exe) spawns a suspicious child process such as cmd.exe or powershell.exe (T1059.001).
5. The spawned process executes discovery commands (e.g., whoami.exe, systeminfo.exe, net.exe, ipconfig.exe) to gather information about the system and network (T1082, T1016, T1033, T1057).
6. The attacker may use system binary proxy execution (T1218) techniques by invoking utilities such as mshta.exe, regsvr32.exe, or installutil.exe to execute malicious code.
7. The attacker establishes persistence on the system, potentially using scheduled tasks (schtasks.exe) or registry modifications (reg.exe).
8. The attacker moves laterally within the network, escalating privileges, and exfiltrating sensitive data, or deploying ransomware.

Impact

Compromised systems can lead to data theft, system disruption, and further propagation of the attack within the network. Successful exploitation of PDF reader vulnerabilities can provide attackers with initial access to the target environment, potentially impacting hundreds or thousands of machines across an organization. The impact can range from minor data breaches to full-scale ransomware deployment, depending on the attacker's objectives.

Recommendation

• Deploy the Sigma rule "Suspicious PDF Reader Child Process" to your SIEM to detect suspicious child processes spawned by PDF readers.
• Enable process creation logging, specifically monitoring for AcroRd32.exe, Acrobat.exe, FoxitPhantomPDF.exe, and FoxitReader.exe spawning command-line interpreters or other suspicious utilities.
• Ensure PDF reader applications are patched to the latest versions to mitigate known vulnerabilities.
• Implement email filtering to block suspicious attachments and educate users about the risks of opening unsolicited PDF files.
• Monitor network connections originating from PDF reader applications for unusual outbound traffic.

Attack Chain

1. Initial access is gained through an unknown method (e.g., exploiting a vulnerability in a different application, or social engineering).
2. The attacker identifies a suitable Adobe process or component to hijack (e.g., AcroRd32.exe, AdobeUpdate.exe).
3. The attacker replaces the legitimate Adobe binary with a malicious executable or modifies an existing Adobe DLL to inject malicious code.
4. The attacker establishes persistence by ensuring the modified Adobe component is launched automatically upon system startup or user login (e.g., via registry keys, scheduled tasks, or startup folders).
5. When the user or system launches the legitimate Adobe application, the malicious code is executed, providing the attacker with control.
6. The attacker uses the compromised process to perform malicious activities such as downloading additional payloads, exfiltrating data, or establishing command and control.
7. The attacker attempts to maintain stealth by masquerading as a legitimate Adobe process and avoiding detection by security tools.

Impact

Successful exploitation of this technique could allow attackers to maintain long-term, persistent access to a compromised system. This could result in data theft, system compromise, and potential disruption of business operations. Due to the trusted nature of Adobe applications, this technique can be difficult to detect and remediate, potentially affecting a large number of users and organizations across various sectors.

Attack Chain

1. The attacker gains initial access to the system, potentially through exploiting a vulnerability or social engineering.
2. The attacker identifies the location of the RdrCEF.exe file within the Adobe Acrobat Reader installation directory (e.g., C:\Program Files (x86)\Adobe\Acrobat Reader DC\Reader\AcroCEF\RdrCEF.exe).
3. The attacker replaces the legitimate RdrCEF.exe file with a malicious executable. This could involve renaming the original file and placing the malicious file in its place, or overwriting the original file directly.
4. The attacker ensures the malicious executable has the same name as the original RdrCEF.exe file.
5. A user launches Adobe Acrobat Reader.
6. The operating system executes the RdrCEF.exe file as part of Adobe Acrobat Reader's startup process.
7. Because the file has been replaced with a malicious executable, the attacker's code is executed.
8. The attacker maintains persistent access to the system and can perform further actions such as deploying malware or exfiltrating data.

Impact

Successful exploitation of this technique allows attackers to maintain persistence on compromised systems. This can lead to the deployment of ransomware, exfiltration of sensitive data, or further exploitation of the system. The severity is low, but impact can be high, if the adversary uses this technique to gain further access to the compromised system.

Recommendation

• Deploy the Sigma rule "Adobe Acrobat Reader Hijack for Persistence" to your SIEM to detect the replacement of the RdrCEF.exe file.
• Monitor file creation events in the Adobe Acrobat Reader installation directories for suspicious executables using Sysmon or another EDR solution.
• Regularly audit file integrity within the Adobe Acrobat Reader installation directory to identify unauthorized modifications.
• Investigate any alerts generated by the Sigma rules or other detection mechanisms to determine if a system has been compromised.